This invention relates to an image-taking lens system for a lens shutter type of cameras, etc. in which viewfinder optical systems are provided separately from image-taking lenses and, more particularly, to a compact zoom lens system which has a high zoom ratio and is reduced in the total length.
A lens shutter type of cameras used today are mainly zoom cameras with built-in lenses having high zoom ratios. Among frequently used zoom lenses, there are the following two types:
(1) a two-unit type zoom lens comprising a first lens unit that is positive and a second lens unit that is negative, and PA1 (2) a three-unit type zoom lens comprising a first lens unit that is positive, a second lens unit that is positive and a third lens unit that is negative. PA1 (3) a three-unit zoom lens comprising a first lens unit that is negative, a second lens unit that is positive and a third lens unit that is negative, as set forth in Japanese Provisional Patent Publication No. 56(1981)-158316 or No. 61(1986)-221791.
Among zoom cameras having high zoom ratio ratios, there are cameras designed to be held in their optical axis direction (just as much binoculars would be held in one hand), which are being increasingly used because of being held stably enough. For these cameras, it is desired to use lenses which are less variable in the total lengths during zooming and can be compactly built in the cameras under every condition from the wide angle to telephoto settings. One typical example of these lenses is:
The zoom lenses (1) and (2) according to the above prior art may be reduced in the total lengths at the wide angle settings, but are increased in the total lengths at the long focal-length (telephoto) settings. This is particularly true for zoom lenses having a zoom ratio as high as 3 or more; that is, there is an appreciable variation in in the total lengths from the short to long focal-length settings. As a result, the mechanism for extending or retracting lenses telescopically causes the lenses to get out of order, having a nonnegligible influence on lens performance.
Furthermore, since these are all of a telephoto type, the more it is intended to obtain a wide field angle, the more difficult it is to make sure of a back focus.
As already stated, the zoom lenses set forth in Japanese Provisional Patent Publication No. 56-158316 and No. 61-221791 are suitable for application to well-holdable cameras, and do not pose such problems as mentioned above. However, they are so designed exclusively for single-lens reflex cameras that their back foci are very long. However, they are still too long in the total lengths to be applied to a lens shutter type of cameras.